The imp, i, is a minor demon in NetHack. It isn't very difficult to kill, even for relatively low level players. Imps tend to taunt you with random demonic maledictions, either randomly when fighting them, or when #chatted to.

... imps ... little creatures of two feet high that could
gambol and jump prodigiously; ...

[ The Charwoman's Shadow, by Lord Dunsany ]

An 'imp' is an off-shoot or cutting. Thus an 'ymp tree' was
a grafted tree, or one grown from a cutting, not from seed.
'Imp' properly means a small devil, an off-shoot of Satan,
but the distinction between goblins or bogles and imps from
hell is hard to make, and many in the Celtic countries as
well as the English Puritans regarded all fairies as devils.
The fairies of tradition often hover uneasily between the
ghostly and the diabolic state.